WO2023227373A1

WO2023227373A1 - Self-mixing interferometry sensor module for authentication, electronic device and method of detecting a fingerprint

Info

Publication number: WO2023227373A1
Application number: PCT/EP2023/062422
Authority: WO
Inventors: Daniel NAJER; Laurent Nevou; Ferran Suarez; Goran Stojanovic; Markus Dantler
Original assignee: Ams International Ag
Priority date: 2022-05-23
Filing date: 2023-05-10
Publication date: 2023-11-30

Abstract

A self-mixing interferometry sensor module (10) for authentication comprises an array (20) of light emitters (21), a detector unit (30) and an electronic processing unit (40). The light emitters (21) of the array (20) are operable to emit coherent electromagnetic radiation out of the sensor module (10); and undergo self-mixing interference, SMI, caused by reflections of the emitted electromagnetic radiation from a finger to be placed outside the sensor module (10), respectively. The detector unit (30) is operable to generate output signals indicative of the SMI of the light emitters (21), respectively. The electronic processing unit (40) is operable to determine from the generated output signals a fingerprint profile of the finger placed outside the sensor module (10).

Description

SELF-MIXING INTERFEROMETRY SENSOR MODULE FOR AUTHENTICATION, ELECTRONIC DEVICE AND METHOD OF DETECTING A FINGERPRINT

The following relates to a self-mixing interferometry sensor module for authentication, an electronic device and to a method of detecting a fingerprint for authentication.

Modern consumer electronics devices, such as cell phones, wearables (like watches, smart glasses, ...) , tablets, personal computers, automotive (driver identification) , fingerprint scanners, or security systems, often contain means for user authentication such as fingerprint sensors for authentication of a user, driver identification, or verification of a credit card signature.

Fingerprint sensors to date are often based on an array of capacitors that sense where fingerprint ridges are in contact with a sensor surface or not, thereby creating a map of the fingerprint. The problem is that the capacitive sensors are sensitive to moisture or dirt and the footprint is relatively big (some 10 mm) . Also, this type of sensor does not work behind a phone screen, for example.

A next generation of fingerprint sensors are "optical" sensors. Optical fingerprint sensors are based on imaging, e.g. taking a picture of the fingerprint with a CMOS camera. The problem is that they are easy to spoof (e.g., by simply showing a picture of the finger/ fingerprint instead of a real finger) .

Another generation of fingerprint sensors is based on ultrasound. Ultrasound sensors may work on any kind of surface and provide an accurate 'map' of the fingerprint ridges, so they are secure. However, this is still a new technology to be proven, but some reports indicate that ultrasound sensors may be prone to failures for recognition and can be slow (some 0.7 s) .

The prior art came up with other solutions, including face recognition (e.g. light structure or time of flight, ToF) , which can serve for cell phone user identification. However, these systems require more complex architectures than other sensors and come along with additional requirements (e.g., the user's face must be facing the sensor, eye safety, etc.) .

Thus, an object to be achieved is to provide a sensor module for electronic devices that overcomes the aforementioned limitations and provides authentication with an improved level of security. A further object is to provide an electronic device comprising such a sensor module and a method of detecting a fingerprint.

These objects are achieved with the subject-matter of the independent claims. Further developments and embodiments are described in dependent claims.

The following relates to an improved concept in the field of optical sensing. One aspect relates to the idea of employing self-mixing interferometry to scan a fingerprint and derive a map of the fingerprint ridges to allow authentication of an electronic device. A sensor module provides fingerprint scanning which is based on the occurrence of self-mixing, or back-injection, interference. At least part of the light emitted by light emitters inside the module is reflected off a target outside the module, e.g. a user's finger, back into a cavity of the light emitters , causing a modulation both in amplitude and in frequency of the emitted optical beam, which in turn also modulates an electronic property of the emitter . In other words , the light emitter itsel f ef fectively becomes a distance sensor as a degree of the modulation depends on these properties of the topography of the target . One advantage compared to traditional sensors is a signi ficantly decreased footprint of the SMI sensor module and lower costs owing to the absence of collimation optics and photodiodes , for example .

In at least one embodiment , a sel f-mixing interferometry sensor module comprises an array of light emitters , a detector unit and an electronic processing unit .

Each light emitter is operable to emit coherent electromagnetic radiation out of the sensor module , e . g . towards an external obj ect . Furthermore , the light emitters are operable to undergo sel f-mixing interference , SMI for short , which may be caused by reflections of the emitted electromagnetic radiation from a finger placed outside the sensor module . The detector unit is operable to generate output signals , which are indicative of the SMI of the light emitters , respectively .

The electronic processing unit is operable to determine from the generated output signals a fingerprint profile of the finger placed outside the sensor module . The fingerprint profile is indicative of a fingerprint of the finger placed outside the sensor module . For example , the finger comprises characteristic friction ridges . The ridges define heights and depths of a finger surface and essentially define a surface profile of the finger . The fingerprint profile determined by the electronic processing unit can be considered a representation or map of the finger surface profile .

For example , the light emitters are arranged to enable sel fmixing interference , and typically comprise a cavity resonator, into which at least a fraction of the light emitted by the light emitters can be reflected, or backscattered, from the finger outside the module . For example , the light emitters are implemented as a laser diode and comprise a laser cavity . The light emitters are configured to emit coherent light , e . g . in an infrared ( IR) , visible or ultraviolet (UV) range of the electromagnetic spectrum, out of the sensor module . For example , the light emitters are configured to generate a continuous emission or to emit light in a pulsed fashion, the latter potentially aiding in achieving an overall reduction in power consumption .

Upon the aforementioned back-inj ection of the emitted light into the cavity and due to the finger surface profile of the finger outside the module , the light is reflected of f di f ferent distances and the light emitters undergo sel fmixing interference which includes information of the di f ferent distances , and, in turn, of the finger surface profile .

For example , when the light emitted and/or the electromagnetic field from the laser cavity is reflected back into the cavity and changes its phase due to target distance changes , it causes a modulation in the amplitude and/or frequency of the solitary laser electromagnetic light field due to an interference process . The sel f-mixing interference generates periodic fringes in the signal of the solitary laser . More accurately, SMI modulates the optical power (which is usually observed by measuring it in a photodetector, photocurrent ) and the threshold laser gain (which can be detected monitoring the laser voltage or laser current ) . Another way of generating SMI is through modulation of the laser emission wavelength, e . g . ramping the laser current periodically (via triangular function current ramp or changing the laser cavity via a MEMS mirror ) .

When no finger is present outside the module in the field of emission of the light source so as to intercept and reflect light of the latter, no sel f-mixing interference occurs within the light emitter .

As discussed above , SMI eventually alters a property of the light emitters . This property is indirectly measured by means of the detector unit , which generates the output signals as a function of said property, or change of said property . The output signals may be measured as current or voltage , for example . Thus , the detector unit may have means , e . g . active or passive circuitry, to measure said change as an electronic property .

The improved concept provides a sensor module or fingerprint sensor which can be used for authentication, such as user authentication . The fingerprint sensor can be incorporated into cell phones , wearables , tablets , personal computers , cars , recognition or biometric sensors , or security identi fication devices . Due to the highly sensitive optical properties of sel f-mixing interferometry, the fingerprint sensor provides unique fingerprint profiles which are very di f ficult to spoof . Using sel f-mixing interferometry, the fingerprint depth can be measured, creating a depth map . The module allows for fast fingerprint detection combining low- cost, low-footprint, high-reliability, and low-power- consumption solutions as light emitters can be used as both source and self-mixing detector.

Due to the low cost, small footprint and low power consumption, the sensor module can be applied to the consumer market (e.g., cell phones or computers) . The module may work behind displays, such as an organic LED cell phone display (BOLED application) , or can be integrated into a display. The detection is largely immune to ambient light due to coherence of light emitted by the module.

In at least one embodiment, the light emitters, the detector unit and the electronic processing unit form an integrated semiconductor device, such as a CMOS integrated circuit device, on a common substrate. In particular, a sensor module according to the improved concept can be free of any dedicated photodetectors for sensing reflected or backscattered light. In addition, or alternatively, the sensor module comprises a sensor package into which the light emitters, detector unit and the electronic processing unit, or the integrated semiconductor device formed by the light emitters, detector unit and the electronic processing unit, are integrated.

An electronic device may use a self-mixing interferometry sensor module as part of detecting a fingerprint with respect to an input surface of the device, e.g. by swiping over or placing the finger on the input surface. Specifically, such a self-mixing interferometry sensor module can be used with a wide range of consumer and other electronic devices. In at least one embodiment , the light emitters comprise semiconductor laser diodes and/or resonant cavity light emitting devices . These devices feature coherent emission to generate SMI fringes . A resonant cavity light emitting device can be considered a semiconductor device which is operable to emit coherent light based on a resonance process . In this process , the resonant cavity light emitting device may directly convert electrical energy into light , e . g . , when pumped directly with an electrical current to create ampli fied stimulated emission .

In at least one embodiment , the light emitters comprise vertical cavity surface emitting laser, VCSEL, diodes . VCSELs are an example of a resonant cavity light emitting device and feature a beam emission that is perpendicular to a main extension plane of a top surface of the VCSEL . The VCSEL diode can be formed from semiconductor layers on a substrate , wherein the semiconductor layers comprise two distributed Bragg reflectors ( DBRs ) enclosing active region layers in between and thus forming a cavity . VCSELs and their principle of operation are a well-known concept and are not further detailed in this disclosure . For example , the VCSEL diode is configured to have an emission wavelength of 940 nm, 850 nm, or another wavelength . The VCSEL diode can be configured to emit coherent laser light when forward biased, for instance .

In at least one embodiment , the sensor module comprises an array of light emitters , which are arranged as a onedimensional array .

The electronic processing unit is operable to determine the fingerprint profile as the finger sweeps at a distance with respect to the module . For example , the sensor module can be placed with respect to a transmissive and/or transparent cover of an electronic device . The cover may serve as a contact surface to place the finger on . The light emitters can be addressed and read out by the detector unit . This way, the output signals can be combined into a partial fingerprint profile , which basically corresponds to a one-dimensional section of the fingerprint . As the finger sweeps along the contact surface , the light emitters can be addressed and read out by the detector unit so as to establish partial fingerprint profiles as a function of time , which correspond to further sections of the fingerprint . The partial fingerprint profiles may be collected by the electronic processing unit and be combined to yield the fingerprint profile . The resulting fingerprint profile is a two-dimensional representation or map of the actual fingerprint . The contact surface may assure a defined distance between the finger and the sensor module .

In at least one embodiment , the array comprises light emitters which are arranged as a two-dimensional array . The electronic processing unit is operable to determine the fingerprint profile as the finger is placed at a distance with respect to the module .

For example , the sensor module can be placed with respect to the transmissive and/or transparent cover or contact surface of an electronic device . The light emitters can be addressed and read out by the detector unit . As the array is two- dimensional , a complete fingerprint profile can be established, without the need to sweep the finger along the contact surface . The fingerprint profile may be determined by combining the output signals into rows or columns of the array, and the electronic processing unit combines the output signals into the fingerprint profile. The resulting fingerprint profile is a two-dimensional representation or map of the actual fingerprint. The contact surface may assure a defined distance between the finger and the sensor module.

For example, there may be different ways to get SMI signals:

1) Relative distances: the finger approaches the laser array, and when it contacts the surface (e.g., in focus) , the SMI signals at different distances change because of the fingerprint ridges, thus allowing to construct a depth map, or

2) Absolute distances modulating, e.g. of laser array current .

Resolution may be different and could be supported by using a tunable laser (e.g., a MEMs mirror laser) .

In at least one embodiment, the detector unit is operable to detect a junction voltage of the light emitters, respectively. In turn, the output signals are generated as a function of said junction voltages, respectively. Junction voltage is one possible electronic property of the light emitters which may change as a result of SMI. For example, the detector unit comprises one or more voltage meters to detect the junction voltage (s) . An addressable array allows for voltage readout. A power readout may rely on an array of light detectors, e.g. to get laser independent signals.

In at least one embodiment, the detector unit is operable to detect an optical power output of the light emitters, respectively. In turn, the output signals are generated as a function of said optical power outputs, respectively. Optical power is another possible property of the light emitters which may change as a result of SMI. For example, the detector unit comprises one or more photodetectors , such as a photodiode , or a photodiode array to detect optical power outputs .

In at least one embodiment , the electronic control unit is further operable to measure from the output signals a displacement of a target within a human body to sense and/or monitor physiological parameters , e . g . to measure the pulse micro-movements to measure the heart rate , or measure the speed of flow of blood particles in vessels close to the skin surface .

For example , some light emitters are configured to illuminate and receive reflections of f the finger surface . Other light emitters , however, can be configured to illuminate and receive reflections of f a target within the finger, e . g . blood particles in vessels close to the skin surface . The di f ferent configurations can be achieved by means of a lens or dedicated optics , for example . As the light emitters can be individually addressed by means of the detector unit , output signals can be assigned either to the fingerprint or the target within the finger . The physiological parameters may be characteristic to a person or user . Thus , the physiological parameters sensed or monitored by the sensor module can be used as an additional level of security for user authentication .

In at least one embodiment , the sensor module further comprises a proximity sensor to trigger a fingerprint mode of operation . The light emitters may only be activated during the fingerprint mode of operation . Said mode of operation may be activated by means of user interaction or be triggered by the proximity sensor when a finger approaches the module or is actually placed on the module.

In at least one embodiment, the sensor module further comprises a lens or an array of lenses combined with the light emitters. The light emitters or array can be combined with a single lens or individual lenses. The lenses can be integrated in the light emitters (e.g. etched in the back of the substrate or be implemented as meta-lenses on the back or in the front of the epitaxy) .

The lens or array of lenses can be arranged to guide emitted light to and collect reflected light from a desired target. For example, the finger can be in focus of the lens or array of lenses. Furthermore, the lens or array of lenses can have a focus which is inside the finger, e.g. to improve detection of physiological parameters.

In at least one embodiment, an electronic device comprises a self-mixing interferometry sensor module according to one of the aforementioned aspects. Furthermore, a housing comprises the sensor module and a transmissive and/or transparent cover. The transmissive and/or transparent cover serves as a contact surface for a user's finger, i.e. the light emitters are operable to illuminate the finger and receive reflections off the finger. The housing is configured to position the light emitters at a distance from the user's finger. For example, the transmissive and/or transparent cover is arranged distant from the light emitters in an emission direction of the light emitters.

Examples of electronic devices include consumer electronics devices, such as cell phones, wearables (like watches, smart glasses, ...) , tablets, personal computers, automotive (driver identification) , fingerprint scanners, or security systems. For example, wearable electronic devices are considered to be any device which by their appearance are wearable and/or which are designed to be worn on the body. A wearable electronic device may be required to be worn to function, e.g. may be conceptually linked to the wearer's body. Some wearable electronic devices may require the user interface to be present and available all the time, others may require no input (such as a wrist unit or chest belt of a heart-rate monitor) . Wearable electronic devices may include a smartwatch or fitness tracker, and the like.

In at least one embodiment, the electronic device comprises a processing unit which is coupled to the sensor module. The processing unit is configured to receive the fingerprint profile from the sensor module and compare the profile with a saved fingerprint profile. Furthermore, the processing unit is operable to compare the received and saved profiles in order to authenticate a user. The processing unit can be a central processing unit, CPU, of the wearable electronic device, or a system-on-a-chip, SOC, that is dedicated to process output signals of the light emitters, for instance.

In at least one embodiment, the electronic device comprises a display. The transmissive and/or transparent cover may be part of the display or be separate from the display. The display may be an OLED or micro-display, for example.

In at least one embodiment, the sensor module is arranged behind the display or integrated into the display. For example, the sensor module can be placed behind the display (BOLED) or be part of a micro-display. Further embodiments of the electronic device become apparent to the skilled reader from the aforementioned embodiments of the sel f-mixing interferometry sensor module , and vice-versa .

Furthermore , a method of detecting a fingerprint is provided, comprising at least the following steps .

One step includes emitting, by means of light emitters , coherent electromagnetic radiation out of a sensor module . Another step includes inducing, within the light emitters , sel f-mixing interference , SMI , caused by reflections of , or scattering by, the emitted electromagnetic radiation from a finger to be placed outside the sensor module . Another step includes generating output signals indicative of the SMI of the light emitters , respectively . Another step includes generating a fingerprint profile from the output signals indicative of a fingerprint of the finger .

These steps can be complemented by further procedural steps , such as determining from the di f ference signal a physiological parameter via the finger, and generating a processed output signal that comprises information of the determined physiological parameter .

Further embodiments of the method become apparent to the skilled reader from the aforementioned embodiments of the sel f-mixing interferometry sensor module and of the wearable electronic device , and vice-versa .

The following description of figures may further illustrate and explain aspects of the sel f-mixing interferometry sensor module , wearable electronic device and the method of detecting movements . Components and parts of the sel f-mixing interferometry sensor that are functionally identical or have an identical ef fect are denoted by identical reference symbols . Identical or ef fectively identical components and parts might be described only with respect to the figures where they occur first . Their description is not necessarily repeated in successive figures .

In the figures :

Figure 1 shows an exemplary embodiment of a sel f-mixing interferometry sensor module , and

Figure 2 shows a theoretical example output signal of a sel f-mixing interferometry sensor module .

The proposed sel f-mixing interferometry sensor module can be used in various electronic devices , including wearable electronic devices , such as smartwatches and the like . For example , the sensor module enables an electronic device (wearable or not ) to derive a fingerprint of the finger of a user by applying light as a stimulus , and detecting a response from the interaction of the stimulus with the fingerprint ridges . In the following, two possible examples are disclosed which can be considered representative for the various possible applications . The examples relate to a onedimensional and two-dimensional array of light emitters .

Figure 1 shows an exemplary embodiment of a sel f-mixing interferometry sensor module for an electronic device . The drawing shows an electronic device 100 , e . g . a cell phone , comprising a sensor module 10 . The sensor module is integrated into and electrically connected to the electronic device . In fact , the electronic device comprises a housing 50 with a transmissive and/or transparent cover 51 . The sensor module is placed or mounted in the housing . The transmissive and/or transparent cover may be part of a display of the electronic device or a dedicated finger contact surface , for example .

The sensor module 10 further comprises an array 20 of light emitters 21 , a detector unit 30 and an electronic processing unit 40 . The sensor module can be implemented as a sensor package , into which the light emitters , detector unit and the electronic processing unit , or the integrated semiconductor device formed by the light emitters , detector unit and the electronic processing unit , are integrated . For example , the detector unit and the electronic processing unit form an integrated semiconductor device , such as a CMOS integrated circuit device , on a common substrate . The light emitters can either be integrated into the integrated semiconductor device or be electrically connected to the integrated semiconductor device as external components .

The light emitters 21 are implemented as vertical cavity surface emitting laser, or VCSEL, diodes . VCSELs are an example of resonant cavity light emitting devices . The light emitters comprise semiconductor layers with distributed Bragg reflectors (not shown) which enclose active region layers in between, thus forming a cavity . The VCSELs feature a beam emission of coherent electromagnetic radiation that is perpendicular to a main extension plane of a top surface of the VCSEL . For example , the VCSEL diodes are configured to have an emission wavelength in the infrared range , e . g . of 940 nm or 850 nm . The sensor module 10 may comprise a laser driver embedded in the integrated semiconductor device as a means to drive the light emitters 21. The light emitters are either implemented as a one-dimensional or as a two-dimensional array, which determines how the sensor module may be operated. Details will be discussed further below. The laser driver allows to address the light emitters so that the detector unit can read out the emitters and generate characteristic output signals.

The detector unit 30 is shown as a schematic building block only. The detector unit comprises means, e.g. active or passive circuitry, to measure an optical or electronic property of the light emitters 21. For example, the detector unit comprises a current or voltage meter to detect a junction voltage of the light emitters, respectively. Junction voltage is one possible electronic property of the light emitters and may change as a result of self-mixing interference. In addition, or alternatively, the detector unit comprises one or more photodetectors, such as a photodiode, to detect an optical power output of the light emitters, respectively. The optical power output is a possible optical property of the light emitters and may change as a result of self-mixing interference. In some embodiments, the photodetectors can be integrated on the epitaxy of the light emitters 21.

The light emitters 21 are combined with optics, such as a lens or lens system, e.g. a meta-lens, etched in the substrate or separate (physical individual lens: micro-optics or lenses) or a common lens or an array of micro-lenses. Focusing is for example needed for sufficient SMI signal strength on a user's finger. The electronic processing unit 40 constitutes a functional unit of the sensor module , which conducts a number of (pre ) - processing steps . Its functionality will be discussed in further detail below . These steps could include counting SMI signal fringes in the temporal space or a fast Fourier trans formation in the frequency space of the SMI signals to generate the corresponding fingerprint profile . For example , the electronic processing unit comprises a microprocessor or AS IC .

The electronic device typically comprises additional components (not shown) , such as a processing unit , to receive the fingerprint profile from the sensor module and conduct a comparison with a saved fingerprint profile to authenticate a user . The processing unit can be a central processing unit , CPU, of the electronic device , microprocessor, or a system- on-a-chip, SOC, which is dedicated to process the fingerprint profile and/or output signals of the light emitters 21 , for instance .

In operation, a finger is placed on the transmissive and/or transparent cover of the electronic device which serves as a contact surface . This way, the light emitters 21 face the finger of the user and irradiate the finger . The light emitters emit coherent light , e . g . in an infrared ( IR) , visible or ultraviolet (UV) range of the electromagnetic spectrum, out of the sensor module and by means of the lens or lenses towards the finger . For example , the light emitters generate a continuous emission or emit light in a pulsed fashion, wherein the latter potentially aids in achieving an overall reduction in power consumption . In sel f-mixing interferometry, a portion of the light emitted by the light emitters 21 is reflected back and coupled into the light emitters , creating interferences therein . A threshold voltage and threshold current of the light emitters , e . g . a laser source , will be af fected by the retro- feedback, so the output laser power changes . The small changes in these parameters can be measured as the sel fmixing signal , denoted output signal , on an emitter per emitter basis .

The changes measured directly in the laser source (voltage or current ) do not require any other sensors . A photodetector can be used to measure the changes in the laser power due to sel f-mixing . The photodetector can be placed close to the laser side by side or behind the laser or integrated in the laser epitaxy . The voltage or power-readout sel f-mixing signal contains information of the distance and the velocity of the target (here : finger ) . The output signals collected from the light emitters essentially map a section of the fingerprint , or a partial fingerprint profile . The fingerprint profile can be constructed in di f ferent ways , depending on whether a one- or two-dimensional array 20 is implemented .

In case of a one-dimensional array, the finger sweeps along the cover at a distance with respect to the sensor module as indicated by arrows in the drawing . The cover may serve as a contact surface to place the finger on . The light emitters 21 can be addressed and read out by the detector unit 30 as a function of time . This way, the output signals are combined into a partial fingerprint profile , which basically corresponds to a one-dimensional section of the fingerprint . As the finger sweeps along the contact surface , the light emitters can be addressed and read out by the detector unit so that the output signals become a function of time . Further partial fingerprint profiles are established, which correspond to further sections of the fingerprint .

The partial fingerprint profiles may be collected by the electronic processing unit and be combined to yield the fingerprint profile . The resulting fingerprint profile is a two-dimensional depth representation or depth map of the actual fingerprint . The contact surface may assure a defined distance between the finger and the sensor module .

In case of a two-dimensional array, the finger rests on the contact surface and the output signals are read out as rows or columns of the array, for example . This way, the electronic processing unit combines the output signals into the fingerprint profile . The resulting fingerprint profile is a two-dimensional depth representation or depth map of the actual fingerprint . In this configuration it is possible to obtain an SMI signal at the moment of a finger touching the contact surface and derive a depth map points per light emitter ( crests and valleys of the finger print ridges ) . Or a light emitter current can be used to dynamically change the emission wavelength and produce an SMI signal related to the absolute distance from light emitter to target , e . g . by using a triangular current modulation ramp .

The sel f-mixing interferometry can measure relative distances with an accuracy of hal f a wavelength of the laser source light . Typical laser sources range from 400 to 2000 nm . Some lasers are close to 1000 nm, for example 850 nm, 940 nm and 980nm in case of VCSELs . So , the resolution is around hal f a micron . Fingerprint ridges vary from person to person, but for a good approximation the height of a ridge could be some 50 pm and the width some 500 pm . Thus , there is room to focus or collimate a laser to scan and have enough resolution to measure the height of the ridges . The fingerprint profile can be determined with a high degree of accuracy and, thus , of fers a spoof proof way to authenticate a user by way of his or her fingerprint .

Figure 2 shows a simulated example output signal of a sel fmixing interferometry sensor module . The graphs include voltage readout and power readout (photodetector photocurrent ) , and a reconstructed topography of a simulated fingerprint scan ridge from the SMI signals . The signals shown in this drawing are for the case of a one-dimensional array with a relative displacement measurement combined with sliding the finger along the contact surface . The graph gl on the top shows the simulated reconstructed topography ( of a finger ) in pm from the SMI signal . The graph g2 in the middle shows a voltage AC signal as one possible SMI output signal ( in mV) as a function of time . The graph g3 on the bottom shows a photocurrent signal ( in mA) as another possible SMI output signal as a function of time . Note that the time dependence is introduced due to sliding or sweeping the finger along the contact surface . The number of fringes involved in a sweep allows to calculate a 2D depth profile . The number of fringes per time gives you the speed at which the user sweeps . The finger is assumed to be at a target distance of 6 mm with respect to the sensor module . Sweeping is at f O = 1 kHz and the ridges are set to 50 pm as an example .

The results simulate an output signal in a light emitter as a function of time . The fingerprint signal can be determined by means of a Fourier trans formation, e . g . a Fast Fourier Trans formation, FFT .

The output signals shown in Figure 2 can be processed by means of FFT or similar trans formations . This can be done for each light emitter along the one-dimensional array . The result of the FFT determines a partial fingerprint profile , which indicates a topography height as a function of space . Taking all determined partial profiles together, a two- dimensional fingerprint profile can be constructed which constitutes a map or depth map of the actual fingerprint This map can be used to authenticate a user, e . g . , by comparison with a saved profile .

The further embodiment of a two-dimensional array relies on placing the finger steadily on top of the array without sliding the finger, where each light emitter measures the depth of the ridges via "absolute" distance measurements . While in this configuration, the resolution can be much lower than with the "relative" displacement measurement and sliding finger using the one-dimensional array, there are ways known from literature to increase the resolution AD in absolute distance measurement . For "absolute" distance measurements , the distance resolution depends on the laser wavelength tuning ( AX) capability given by the following formula AD = X²/ ( 2 *AX) , where X is the laser wavelength . As an example : X = 940 nm and AX = 0 . 4 nm (VCSEL ) maximum tuning range by varying the VCSEL driving current , the AD is in the order of mm . However, there are methods mentioned in literature that allow to signi ficantly increase this resolution towards 10 pm . Another possibility is to increase the AX, e . g . via a tunable-cavity or MEMS-mirror laser . In conclusion, the disclosure provides a fingerprint sensor which comprises an array of light emitters , which can be collimated or focused on a transparent surface , where the user' s finger is swept or placed . The light emitters are aligned in a sel f-mixing configuration to allow part of the reflected light to be coupled back into the laser cavity . For example , during the sweep, the fingerprint ridges change the laser-target distance and the phase of the signal reflected back produces the corresponding sel f-mixing interference signal as an output signal .

The light emitters can be a laser source arranged as an array of lasers ( e . g . ID or 2D addressable VCSEL array) . The lasers or laser array can be combined with a single lens or individual lenses . The lenses can be integrated in the laser ( e . g . etched in the back of the substrate or meta-lenses on the back or in the front of the epitaxy) .

The sensor module can be combined with a proximity sensor to trigger the fingerprint sensor when the finger is in a close range . The fingerprint sensor can be placed behind a display (BOLED) or be part of a micro-display . The sensor module can have additional capabilities like heartbeat pulse recognition and variability, blood flow and relative oxygen content or other biometric sensitivity .

The improved concept provides a fingerprint scan as a biometric means for personal identi fication due to the unique personal characteristics . To increase security, a depth map of the fingerprints is required so that the sensor module cannot be spoofed easily . Phase locked interferometry for surface profile measurements can be used to obtain resolution below the micron range . While this speci fication contains many speci fics , these should not be construed as limitations on the scope of the invention or of what may be claimed, but rather as descriptions of features speci fic to particular embodiments of the invention . Certain features that are described in this speci fication in the context of separate embodiments can also be implemented in combination in a single embodiment . Conversely, various features that are described in the context of a single embodiment can also be implemented in multiple embodiments separately or in any suitable subcombination . Moreover, although features may be described above as acting in certain combinations and even initially claimed as such, one or more features from a claimed combination can in some cases be excised from the combination, and the claimed combination may be directed to a sub-combination or variation of a sub-combination .

Similarly, while operations are depicted in the drawings in a particular order, this should not be understood as requiring that such operations be performed in the particular order shown or in sequential order, or that all illustrated operations be performed, to achieve desirable results . In certain circumstances , multitasking and parallel processing may be advantageous .

This patent application claims the priority of German patent application 102022112919 . 5 , the disclosure content of which is hereby incorporated by reference . References

10 sensor module

20 array 21 light emitters , e . g . VCSELs

22 lens

30 detector unit

40 electronic processing unit

50 housing 51 transmissive and/or transparent cover

100 electronic device gl graph g2 graph g3 graph

Claims

1. A self-mixing interferometry sensor module (10) for authentication, comprising an array (20) of light emitters (21) , a detector unit (30) and an electronic processing unit (40) , wherein :

- the light emitters (21) of the array (20) are operable to emit coherent electromagnetic radiation out of the sensor module (10) ; and undergo self-mixing interference, SMI, caused by reflections of the emitted electromagnetic radiation from a finger to be placed outside the sensor module (10) ;

- the detector unit (30) is operable to generate output signals indicative of the SMI of the light emitters (21) , respectively;

- the electronic processing unit (40) is operable to determine from the generated output signals a fingerprint profile of the finger placed outside the sensor module (10) .

2. The sensor module according to claim 1, wherein the light emitters (21) comprise:

- semiconductor laser diodes,

- resonant cavity light emitting devices, and/or

- vertical cavity surface emitting laser, VCSEL, diodes.

3. The sensor module according to claim 1 or 2, wherein

- the array (20) comprises light emitters (21) arranged as a one-dimensional array, and

- the electronic processing unit (40) is operable to determine the fingerprint profile as the finger sweeps at a distance with respect to the module.

4. The sensor module according to claim 1 or 2, wherein

- the array (20) comprises light emitters (21) arranged as a two-dimensional array, and

- the electronic processing unit (40) is operable to determine the fingerprint profile as the finger is placed at a distance with respect to the module.

5. The sensor module according to one of claims 1 to 4, wherein the detector unit (30) is operable to:

- detect a junction voltage of the light emitters (21) , respectively, and

- generate the output signals as a function of said junction voltages, respectively.

6. The sensor module according to one of claims 1 to 5, wherein the detector unit (30) is operable to:

- detect an optical power output of the light emitters (21) , respectively, and

- generate the output signals as a function of said optical power outputs, respectively.

7. The sensor module according to one of claims 1 to 6, wherein the electronic processing unit (40) is operable to measure from the output signals a displacement of a target within a human body to sense and/or monitor physiological parameters .

8. The sensor module according to one of claims 1 to 7, further comprising a proximity sensor to trigger a fingerprint mode of operation.

9. The sensor module according to one of claims 1 to 8, comprising a lens (22) or an array of lenses combined with the light emitters (21) .

10. The sensor module according to one of claims 1 to 9, wherein the light emitters, the detector unit and the electronic processing unit form an integrated semiconductor device .

11. An electronic device (100) comprising:

- a self-mixing interferometry sensor module (1) according to one of claims 1 to 10, and

- a housing (50) comprising the sensor module (10) and a transmissive and/or transparent cover (51) to be arranged on a user's finger, wherein the housing (50) is configured to position the light emitters (21) at a distance from the user's finger .

12. The electronic device (100) according to claim 11, further comprising a processing unit which is coupled to the sensor module, wherein the processing unit is configured to receive the fingerprint profile from the sensor module and compare the profile with a saved fingerprint profile, and issue an authentication based on the comparison of the received and saved fingerprint profiles.

13. The electronic device (100) according to claim 12, further comprising a display, wherein the transmissive and/or transparent cover (51) is comprised by the display or separate from the display.

14. The electronic device (100) according to claim 13, wherein the sensor module (10) is arranged behind the display or integrated into the display.

15. A method of detecting a fingerprint, comprising the steps of :

- emitting, by means of light emitters (21) , coherent electromagnetic radiation out of a sensor module (10) ;

- inducing, within the light emitters (21) , self-mixing interference, SMI, caused by reflections of the emitted electromagnetic radiation from a finger placed outside the sensor module (10) ; - generating output signals indicative of the SMI of the light emitters (21) , respectively; and

- generating a fingerprint profile from the output signals indicative of a fingerprint of the finger.